1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more particularly to CMOS integrated circuits with p type and n type nonplanar transistors with metal gate electrodes and methods of fabrication.
2. Discussion of the Related Art
In order to increase device performance, silicon on insulator (SOI) transistors have been proposed for the fabrication of modern integrated circuits. FIG. 1 illustrates a standard fully depleted silicon on insulator (SOI) transistor 100. SOI transistor 100 includes a single crystalline silicon substrate 102 having an insulating layer 104, such as a buried oxide formed thereon. A single crystalline silicon body 106 is formed on the insulating layer 104. A gate dielectric layer 108 is formed on the single crystalline silicon body 106 and a gate electrode 110 formed on the gate dielectric 108. Source 112 and drain 114 regions are formed in the silicon body 106 along laterally opposite sides of a polysilicon gate electrode 110.
Fully depleted SOI have been proposed as a transistor structure to take advantage of ideal sub-threshold gradients for optimized on current/off current ratios. In order to achieve ideal subthreshold gradients with transistor 100, the thickness of the silicon body 106 must be about ⅓ the size of the gate length (Lg) of the transistor or Tsi=Lg/3. However, as gate lengths scale especially as they approach 30 nm, the need for ever decreasing silicon film thickness (TSi) makes this approach increasingly impractical. At 30 nanometer gate length, the thickness required of the silicon body is thought to need to be less than 10 nanometers, and around 6 nanometer for a 20 nanometer gate length. The fabrication of thin silicon films with thicknesses of less than 10 nanometers, is considered to be extremely difficult. On one hand, obtaining wafer uniformity on the order of one nanometer is a difficult challenge. On the other hand, to be able to contact these thin films to form raised source/drain regions to decrease junction resistance, becomes almost impossible since the thin silicon layer in the source/drain regions becomes consumed during the gate etch and various cleans following the gate etch and spacer etch leaving insufficient silicon 106 for epitaxial silicon to grow on.
A double gate (DG) device, such as shown in FIGS. 2A and 2B, have been proposed to alleviate the silicon thickness issue. The double gate (DG) device 200 includes a silicon body 202 formed on an insulating substrate 204. A gate dielectric 206 is formed on two sides of the silicon body 202 and a polysilicon gate electrode 208 is formed adjacent to the gate dielectric 206 formed on the two sides of the silicon body 202. A sufficiently thick insulating layer 209, such as silicon nitride, electrically isolates the gate electrode 208 from the top of silicon body 202. Double gate (DG) device 200 essentially has two gates, one on either side of the channel of the device. Because the double gate device 200 has a gate on each side of the channel, thickness (Tsi) of the silicon body can be double that of a single gate device and still obtain a fully depleted transistor operation. That is, with a double gate device 200 a fully depleted transistor can be formed where Tsi=(2×Lg)/3. The most manufacturable form of the double gate (DG) device 200, however, requires that the body 202 patterning be done with photolithography that is 0.7× smaller than that used to pattern the gate length (Lg) of the device. In order to obtain high density integrated circuits, it is generally desirable to have the most aggressive lithography occur with respect to the gate length (Lg) of the gate electrode 508. Although, double gate structures double the thickness of the silicon film (since there now is a gate on either side of the channel) these structures, however, are extremely difficult to fabricate. For example, silicon body 202 requires a silicon body etch which can produce a silicon body 202 with an aspect ratio (height to width) of about 5:1.
Another problem associated with transistors 100 and 200 shown in FIG. 1 and FIGS. 2A and 2B, is that the gate electrodes are typically formed from a doped polycrystalline silicon film. Polysilicon gate electrodes suffer from the formation of charge carrier depletion regions also known as “poly depletion”. That is, when a voltage is applied to the polycrystalline gate electrode, a depletion region 120 and 220 forms in the lower part of the polycrystalline gate electrode adjacent to the gate dielectric layer 108 and 206 respectively. The result in affect is an increase in the electrical thickness of the gate dielectric layer. For example, in order to fabricate a transistor, such as shown in FIG. 1, with a 90 nanometer gate length, a 14Å thick silicon oxide dielectric layer is necessary for optimal electrical performance. However, in such a device, the poly depletion region 120 can be on the order of 5Å thereby essentially increasing the electrical thickness (TOx) of the gate dielectric layer by 33%. Such an increase in the gate dielectric electrical thickness dramatically reduces the performance of the fabricated transistor. It is to be appreciated, that as device dimensions are scaled down, in order to integrate an ever larger number of transistors into a single integrated circuit in the electrical thickness of the gate oxide layer must also be proportionally scaled down. Poly depletion effects hinder the ability to further scale down transistor dimensions.